A conventional surface emitting visible light emitting diode(hereinafter also referred to as `surface emitting visible LED`) generally has a top electrode with a square or circular shape which is located at the center of the light emitting surface of LED. In the conventional surface emitting visible LED, the external quantum efficiency is dependent on the structure of the LED and the shape of an electrode for spreading current into the junction. To enhance the light output, the electrode area needs to be as small as possible and the light emitting surface needs to be as wide as possible.
Though the conventional surface emitting visible LED allows easiness in fabrication, it causes the disruption of a gaussian type beam output, broadening the beam with light emission concentrated around the electrode. The emitted light intensity gets decreased as the distance from the electrode gets increased. This is because the current is almost centrally crowded and is not distributed far away from the electrode. The light intensity achieved is directly proportional to the current density obtained at the point. This type of LED with broad light beam cannot be implemented in the optical data-link system, because the coupling efficiency even with a larger core fiber is low due to its beam divergence.
For example, known is a conventional surface emitting visible LED, which comprises a n-type buffer layer, a n-type cladding layer, an active layer, a p-type cladding layer, a current spreading layer and a p-type cap layer which are in turn grown on a n-type GaAs substrate. On the cap layer, an electrode composed of square or circular shaped metal is formed.
Observing an approximated near field pattern of the light emitted from the light emission surface of the conventional surface emitting visible LED, it is proved that the light intensity is maximum around the electrode and gets lowered at the position far away from the electrode, which indicates that the spreading of current is mainly crowded under the electrode contact. Thus, the light output obtained could not be enhanced as expected. To avoid this current crowding around the electrode, a blocking layer is conventionally employed which is formed prior to the growth of the current spreading layer.
However, this type of LED needs the two step layer growth, which needs further time and increases the production cost. In addition, the darkness at the center portion due to the circular shaped electrode reduces the coupling efficiency even with a larger core fiber.
Many papers and Japanese patent applications disclose concerning III-V semiconductors based visible LED which has a wavelength range from 580 to 670 nm. In every case, the electrodes for current spreading have a square or circular shape and are located at the center of LED. One typical example is disclosed in Sugawara et al., Japan Journal of Applied Physics, part 1, Vol.1, No.8, pp.2446-2451. In this report, a cross shaped electrode is used in the top emitting LED for spreading the current. A blocking layer is also used to prevent the current from crowding under the contact. The light output achieved by the cross shaped electrode is still below the level necessary for the optical data link system. To increase the current spreading outside the contact, a thick window layer with a low resistivity is needed. However, even with this thick window layer, the current spreading outside the contact is limited up to a certain level and the light intensity is not enough for practical application.
This type of LED may have no problem for use in the outdoor application. However, in an application such as a short distance data link system, especially based on POF, the use of this conventional LED with the square, circular or cross shaped electrode usually exhibits low coupling efficiency to be completely impractical in the POF based communication system. For the POF based data link system, it would be highly desirable to design a LED which exhibits not only high brightness but also high coupling efficiency.
Japanese patent application laid-open No.2-174272(inventor: Kim) discloses a high brightness LED. In this LED, the brightness is enhanced by employing a n-p-n-p structure under a contact, since the current crowded under the contact can be spread to the light emitting surface. The main drawback of this LED is that prior to making a p-contact a mesa structure up to an active region is formed in the light emitting surface to make a current path and also dopant-like zinc(Zn) is diffused at a high ambient temperature. The mesa formation up to the active region and also the post growth high temperature for Zn diffusion may have an influence on its performance characteristics. As the emitting surface has a high p-doping a large fraction of the light(depending on the light energy) might be absorbed in the emitting surface. In fabrication of this type of LED, it needs to go through several processes to make the LED fabrication cost higher. Further, it is difficult to make this type of LED high speed because of the large capacitance induced due to the contact area which is proportional to the LED size.
Japanese patent application laid-open No.3-190287(inventor:Tajiri) also discloses a high brightness LED array. In this LED, to facilitate the wire bonding, a contact is made on the upper layer following the mesa formation. In this case, the shape of the contact is changed to achieve a high light output. Since this device is closely related to a LED array and the mesa structure is needed prior to the formation of electrode, there will be difficulties to form the electrode unless a thick contact region is provided. Therefore, the cost of LED fabrication increases and it is completely impractical for a single LED to be made.
Japanese patent application laid-open No. 4-174567(inventors: Kato et al.) discloses a high brightness LED array for the use of a printer. This LED is provided with a bottom distributed Bragg reflector(DBR) for reflecting the light returned from a substrate. In this case, the same idea as in the fabricating of a surface emitting laser is implemented. In this DBR, pairs of GaAs/AlAs were used for reflecting 880 nm wavelength light emitted towards a substrate. The type and number of the pairs in this DBR are dependent on the wavelength of output light and this type of DBR cannot be used in a visible LED where 600 to 650 nm wavelengths are to be considered. This type of LED array could be used for a printer, but it cannot be implementable in another application such as in a data link system. This is because the shape of a top electrode is the same as that of the previously mentioned conventional LED therefore resulting in providing a very low coupling efficiency to a fiber.
Japanese patent application laid-open No. 4-259263(inventors: Nitta et al.) discloses a InAlGaP based visible LED. In this LED, to avoid the Zn diffusion into an active region, an additional layer of InGaP is employed which has a bandgap greater than that of the active region layer. No additional layer for current spreading is used. However, as described previously, this type of visible LED with a circular shaped electrode centrally located is not useful in a short distance data link system.
As explained above, the conventional LEDs have drawbacks that a sufficient light output and coupling efficiency cannot be obtained in the short distance data link system, especially in the POP based data system.